Visual Acuity - Optometry in Practice

Optometry in Practice Vol 5 (2004) 53–70
Visual Acuity
AJ Jackson 1 PhD MCOptom and IL Bailey 2 OD MS FCOptom FAAO
1 2 Department of Ophthalmology, Royal Group of Hospitals, Belfast and School of Optometry, University of Berkeley,
CA, USA
Accepted for publication 14 January 2004
Almost 150 years after Herman Snellen (Snellen 1862,
1864), the renowned ophthalmologist from the University
of Utrecht, published his table of optotypes for assessing
the visual status of the eye, we continue to utilise optotype
test charts as the mainstay of our clinical evaluation of
vision. Although the term ‘visual acuity’ is almost
synonymous with the assessment of central visual status,
using optotypes, it is however more specific and refers to
the ability of the visual system to resolve spatial detail. It
is a measure of the angular size of detail that is just
resolvable by the observer and its limitations are imposed
by a combination of optical and neural factors.
Optical Limitations
When the emmetropic eye is in optimal focus, the image
of a point object, formed at the macula, is not a perfect
point, but a blur circle surrounded by a series of faint
concentric rings (Figure 1). This is a diffraction
phenomenon, which was first satisfactorily explained by
the Astronomer Royal Sir George Airy, in 1835, and
applies to all aperture-limited optical systems (Airy 1834,
cited in Jenkins & White 1976). The central circle is
known as the Airy disc, and its peak intensity is 57 times
Address for correspondence: Prof AJ Jackson, Low Vision Clinic, Department of
Ophthalmology, Royal Group of Hospitals, Belfast BT12 6BA
© 2004 The College of Optometrists
53
Figure 1 Diagrammatic illustration of the diffraction
phenomenon described by Sir George Airy in 1835. The
graph illustrates the relative brightness and position of
surrounding rings in relation to the central disc (Table 1)
(courtesy of Mr C Wolsey).
Table 1. Relative intensities and sizes of the light distribution in the Airy disc and surrounding rings for the
image of a point source
Relative intensity Relative intensity Radius of ring
(L max ) (L total) (radians)
Airy disc 1 1 –
First dark ring 0 0 1.22 λ/p
First bright ring 1/57 1/12 1.64 λ/p
Second dark ring 0 0 2.23 λ/p
Second bright ring 1/240 1/30 2.67 λ/p
L max , relative brightness of surrounding rings at their brightest point, in comparison to the central point in the Airy disc which has been
allocated an arbitrary value of 1; L total , relative brightness of each ring in total compared to the brightness of the central disc.
Relative Brightness
1.22λ/p
1.64λ/p
2.23λ/p
2.67λ/p
Distance

AJ Jackson & IL Bailey
brighter than that of the first surrounding ring. The angular
size, or radius, of each of the surrounding dark and light
rings is a function of the wavelength of the incident light (λ)
and the pupil diameter (p). In the case of the first dark ring
the diameter, expressed in radians, is 1.22 λ/p, where λ and
p are normally expressed in millimetres. The diameter of
the first bright ring is 1.64 λ/p radians. In converting the
measurement from radians to degrees and subsequently to
minutes of arc, one needs to multiply the figure by 57.29
and thereafter by 60 (Table 1).
If the eye were perfectly focused and free of aberrations,
the only optical factor limiting the resolution of the eye
would be diffraction. Applying the Rayleigh criterion, the
amount of overlap between adjacent blur circles, to render
resolution impossible, would be half the diameter of the
blur circle. At this point, the centre of one blur circle
would be aligned with the edge of its neighbour. An eye
with a pupil diameter of 4.6mm would thus have a
minimal angle of resolution (MAR) of 0.5min of arc
whereas an eye with a pupil diameter half that size
(2.3mm), would have an MAR of 1min of arc (Bailey
1998). The relationship between pupil size and image
quality is, however, more complex because superimposed
on diffractive blur are degradations in image quality due to
optical aberrations. These optical aberrations result from
the shape, alignment and positioning of the various optical
surfaces within the eye and variations in the refractive
indices and the chromatic dispersion of the optical media.
When the eye views a point source, it receives a cone of
diverging rays and the diameter of the incident cone is
limited by pupil size. Because of aberrations, the cone of
rays that converge to form the image on the retina has an
apex that is imperfectly defined. Ideally all rays would
converge as a cone to a single point, and with the eye in
focus, the image point at the apex of the image cone would
be on the retina. In reality, for light of a given wavelength,
rays from different parts of the pupil deviate from the ideal
cone pattern, so the image on the retina is a patch rather
than a point. The light distribution across the patch is
called the point spread function and the distribution is
not necessarily symmetrical. In general, rays from the
more peripheral parts of the pupil are more aberrated.
Retinal cones are, however, directionally sensitive, being
less sensitive to rays that fall upon them obliquely (Stiles
& Crawford 1933). The Stiles–Crawford effect thus
reduces the impact of the aberrational effects associated
with rays from more peripheral parts of the pupil. If the
object is a fine line, the cross-sectional distribution of light
in the image has a bell shape. This is called the line spread
function. This profile differs from the point spread
function in that it is narrower and it does not have the
noticeable rings or fringes.
54
For light of different wavelengths, there will be a
longitudinal difference in the position of best focus. Shortwavelength
light will have its best focus about 0.5mm
anterior to the best focus for long-wavelength light. This
represents about 1.5D of longitudinal chromatic
aberration. The light spread in the retinal image depends
on pupil size. Light spread due to diffraction increases as
the pupil becomes smaller. On the other hand, light spread
due to monochromatic and chromatic optical aberrations
is reduced as the pupil becomes smaller. The optimal pupil
size required to balance these effects best is
approximately 2.5mm (Westheimer 1964).
Neural Limitations
A significant neural limitation on visual resolution is
imposed by the anatomical structure of the retinal
photoreceptor layer. In the foveal region, cones are packed
closely together and cones are known to have an average
diameter of about 2µm. It might be expected that, in order
for images of two-point objects to be resolved, they should
fall on two individual cones separated by a cone which is
not stimulated. The two-point images should thus be
separated by a distance of 4µm. Assuming the nodal point
of the eye is 16.67mm from the retina, then the neural
limit to resolution becomes 0.82min of arc. Other neural
limitations of the system may also be imposed by the
various complex interconnections and interactions
between neurones within the retina and visual pathways.
Defining Resolution
Resolution can be defined as minimum detectable,
minimum separable or minimum recognisable.
Minimum detectable resolution
Minimum detectable (or minimum distinguishable)
resolution is the minimum angular size of a spot or the
minimum angular width of a line necessary for it to be
detected against its background. In the case of a spot,
when the diameter of the image formed on the retina
becomes smaller than the surface area of a single
photoreceptor, detection remains possible as long as the
visual system can recognise that adjacent photoreceptors,
one receiving the stimulus and one not, have responded
differently. Threshold detection is achievable when the
spot is large enough or has sufficient contrast to create a
detectable elevation or reduction in illumination on the
photoreceptor, in comparison to its neighbour. Under
photopic conditions, the differential contrast threshold

emains a constant ratio, at approximately 1%,
irrespective of overall field luminance. This phenomenon
forms the basis of Weber’s law (Weber 1834). As lighting
levels drop, the threshold contrast ratio between target
spot and its surround must increase by up to 20 times in
order to enable target detection to take place.
Detectability will also vary as a function of target size,
wavelength and the distribution of luminance within the
visual field. The minimal dimension of a high-contrast
point object required for detection is reported to be
between 10 and 35 seconds of arc, whereas for a highcontrast
line target, the width only needs to be 4–10
seconds of arc (Duke-Elder 1937).
Minimum separable resolution
Minimum separable resolution is a measure of the
minimum separation between adjacent spots or lines
required for the observer to identify the objects as
separate. This form of resolution is usually assessed using
grating targets, the dimensions of which may be expressed
in cycles per degree (cpd). In a grating with a spatial
frequency of 30 cpd, the width of each dark stripe and
each bright stripe will be 1 min of arc. The limit of an eye’s
ability to resolve a high-contrast grating, and distinguish it
from a uniform field of the same average luminance, is a
measure of its threshold spatial frequency for minimum
separable resolution. An eye that can just resolve a 30-cpd
grating can be said to have a MAR of 1 min of arc.
In measuring resolution with grating targets, there may be
spurious resolution, in which the presence of a grating can
be recognised, even though the perception of the number
of stripes in the grating is erroneous. This can be seen most
easily in resolution tests that use sets of three line gratings
that have different spatial frequencies (Figure 2).
Hyperacuities
To this point we have described visual acuity as a measure
of the ability of the eye to detect spatial separation
between details, and that the magnitude of visual acuity is
closely related to the angle between adjacent retinal
receptors. There are, however, some tasks in which
information about location is pooled across many
receptors, allowing spatial discriminations that are finer
than the minimal separable resolution by a factor of about
10-fold. The term ‘hyperacuity’ is used to describe these
ultrafine spatial discriminations. Vernier acuity and
stereoacuity are the two best known of the hyperacuities.
When assessing Vernier acuity, the task is to judge
whether two line segments are in line with each other.
When assessing stereoacuity, the task is to judge whether
one line segment is closer or further away than another in
55
Visual Acuity
Figure 2 Resolution and spurious resolution
An illustration of the light distribution within a three-line
target when the spacing is close to the resolution limit. The
figure depicts the light distribution on the retina viewing an
object that consists of three fine white lines. Due to
diffraction and aberrations, each individual fine line creates
an image with a bell-shaped light profile, and each of these
light distributions is called a line spread function (LSF).
Diffraction and aberrations affect the width and shape of the
LSF.
Bottom: the light distribution of the retinal image when there
is only marginal overlap of the adjacent LSF. The observer
should easily be able to tell there are three lines present.
Middle: the lines have been brought closer together (spatial
frequency has been increased) so there is substantial overlap
between the three LSFs. In the total light distribution of the
image, there are three peaks corresponding to the positions of
the three lines in the object. There is only a relatively small
difference between the retinal illuminance at the three peaks
and the two troughs. If this difference (contrast) is sufficient,
the observer will perceive a low-contrast grating pattern from
the three lines. As the lines are brought a little closer together,
the difference between the retinal illuminance at the peaks
and troughs will be further reduced.
Top: the lines are even closer together. The overlap between
adjacent LSFs is more substantial. Now, summation between
the sloping sides of adjacent LSFs gives rise to peaks in the
total light distribution. Here, the sum of the three LSFs gives
two peaks with an intervening trough. The observer may
perceive a striped pattern of low contrast, but it will appear
that there are only two lines present . This is called spurious
resolution. The observer can tell the object has a grating
structure, but has an erroneous perception of the number of
component lines.

AJ Jackson & IL Bailey
three-dimensional space. Thresholds for these
hyperacuity tasks are commonly in the order of 5 seconds
of arc. Other hyperacuity tasks include recognising that a
line is curved rather than straight, discriminating
deviations from vertical or horizontal, discerning
deviations from parallelism, detecting a bump in a line and
judging equality of lengths in a bisection task.
Minimum recognisable resolution
Minimum recognisable (or minimum legible) is the term
given to the measurement of acuity using optotype
symbols. Whereas grating acuity is often used to
determine optical performance under laboratory
conditions, most clinical tests of visual resolution utilise
symbols or optotypes as targets (Figure 3). The optotypes
chosen may, however, be simple or complex. Landolt
rings, which represent the simplest of targets, were
adopted in 1909 by the XI International Ophthalmological
Congress as the standard test object (Landolt 1899,
Woodruff 1947). Each ring is constructed so that its
external diameter is five times greater than its stroke
width, which in turn, is equal in size to the gap in the ring.
Most vision tests using Landolt rings simply require the
observer to identify the location of the gap which may be
in one of four positions – to the right, left, up or down.
Sometimes obliquely oriented gaps (45º, 135º, 225º and
315º) are included, giving a choice of eight gap
orientations. When testing with Landolt rings, the patient
need not be familiar with any particular alphabet. The test
is therefore well-suited for testing persons who are
illiterate or who cannot name letters that might otherwise
be used as the optotype. For research purposes, Landolt
rings may be the preferred optotype because the
resolution task is essentially the same for each of the
alternative gap positions. When using a combination of
letters as optotypes, the observer’s task and decisionmaking
strategies might be quite different from one letter
to another. Tumbling E (or ‘illiterate E’) charts have been
constructed using an E optotype with four alternative
orientations. Typically, the E is constructed on a 5 × 5 grid
with each of the three parallel limbs being one grid-unit
wide. As for the Landolt ring test, the observer’s task is to
identify the orientation of each optotype by indicating if
the ‘legs’ of the E point right, left, up or down. Forcedchoice
methodology is often used in the assessment of
threshold acuity using tumbling Es or Landolt rings,
especially when used for research purposes.
More traditional visual acuity tests utilise charts with sets
of alphanumeric symbols as optotypes, as this makes the
process of testing simpler and quicker. Individual letter, or
number, optotypes are usually constructed using either a
56
Serif E 5 × 5 Landolt ring
Sans serif H 5 × 5 Sans serif E 5 × 4
Figure 3
An illustration of the font styles and grid structure of serif
and sans serif letters, the Landolt ring and tumbling E.
5 × 4 grid, in which stroke width measures one-fifth of the
grid height, or a 5 × 5 grid. Snellen’s original charts used
letters the stroke width of which were equal to one-fifth of
the letter height. Although most letters were constructed
using a 5 × 5 grid, some 5 × 6 letters were used (Snellen
1862, 1864). His letters had serifs, which are short lines
added to the ends of the limbs of the letters. Since Snellen,
there have been numerous variations in the design of the
letters and the chart layout. Bennett (1965) reviewed the
history of visual acuity chart design at about the time the
British Standards Institute chose to adopt a set of 10
letters based on a 5 × 4 grid format as the standard
optotypes for clinical testing of visual acuity (BSI 4274-1
1968). The UK standards have recently been updated in
such a manner as to introduce log MAR notations and
several other features consistent with log MAR testing
strategies (BSI 4274-1 2003).
Optotype Test Chart Design
Since the time of Snellen, the practice of expressing
central visual status using measurements obtained from
optotype charts has been confounded by the proliferation
of charts utilising different design features. The

assessment of visual acuity using optotype targets has
become almost synonymous with clarity of vision (Figure
4). Bennett (1965) in his extensive review on ophthalmic
test types classified the three principal problems
associated with standardisation as:
1. Selection and style of symbols
2. Progression of sizes of symbols
3. The notation used for expressing acuity
Figure 4 In popular perception the test letter chart quickly
became synonymous with the sight test, as demonstrated by
the cartoonist Sidney Strube (1891–1956) in this newspaper
drawing from 1938. The Prime Minister, Neville
Chamberlain, is portrayed as an ophthalmic optician whose
sum total of equipment consists of an electrically
illuminated screen chart, a simple drop-cell trial frame and
case of trial lenses. The inability of the governing MPs
(portrayed as the British lion in a school cap) to read the
smallest lettering is intended as a symbol for their failure to
focus on what supporters of Appeasement felt to be a more
important issue in a country still recovering from the effects
of the great economic slump. Mother, in the form of Winston
Churchill, dressed somewhat ridiculously as Britannia, sits
in on the test in a position of splendid isolation across the
room. All may not be as it seems, however. As Strube was a
fanatical anti-Nazi and favourite of Churchill it is possible
that the artist’s intention is to illustrate the priority of the
more easily seen letters (courtesy of Mr Neil Handley MA
AMA, Curator British Optical Association Museum, College
of Optometrists, London).
© The Daily Express
The original sketch for this cartoon, in which Chamberlain
is described as an ‘oculist’, is in the collection of the BOA
Museum at the College of Optometrists.
57
Symbol selection
Visual Acuity
Not all letters are of equal legibility and legibility will differ
according to font style. Snellen’s charts (1862) used
‘Egyptian paragon’ styled serif letters, each of which had
prominent ornamental cross-strokes at the end of each
limb. Green (1868, 1905), who favoured simpler font
styles, initially used a sans serif Gothic design and
excluded letters with oblique lines (Z, N, S etc). Hartridge
and Owen, who compared the legibility of Snellen and
Green style letters, recommended that acuity should be
assessed using a series of nine letters (D F H N P T U X Z)
selected from the Green series (Hartridge & Owen 1922).
By 1872, Green’s thoughts on the matter had changed in
favour of a modified serif design as sans serif letters had
been criticised as being ‘unfinished in appearance’ (Green
1872). Dennett (1885) adjusted individual letter sizes in
an attempt to compensate for relative legibility whereas
others, including Cowan (1928) and Monoyer (1875),
emphasised the importance of individually selecting font
styles and linear dimensions (ie 5 × 5, 5 × 4 or 5 × 3) on a
letter-by-letter basis. As suggestions concerning letter
shape and size proliferated, reverse contrast was proposed
as a method of compensating for legibility differences
(Walker 1942). Much of the original work on relative
legibility was undertaken by Woodruff (1947) and Coates
(1935) who accumulated considerable normative data on
legibility. The effect of font style on relative legibility has
been reviewed by Borish (1970). Bennett (1965)
illustrated how different letter design styles could affect
legibility, serif letters in particular, introducing
characteristic identification errors. In his comprehensive
review on the subject, he outlined the rationale used by
various test chart designers for selecting letters of various
styles. British Standard BS4274-1, which was published in
1968, recommends 10 (5 × 4) sans serif letters giving
legibility ratings ranging from 0.92 (R) to 1.10 (Z) (D E F
H N P R U V Z) (BSI 4274-1 1968). The most recent
version incorporates log MAR notation, recommends a
5 × 5 letter grid format, a modification to row progression,
and introduces the letters C and K to the recommended
range from which letters should be selected (BSI 4274-1
2003). In the USA Sloan, who pioneered much of the work
on standardisation, selected a sans serif style (Sloan 1951,
Sloan et al. 1952). Her preference was for 10 letters in a 5
× 5 grid style (S O C D K V R H N Z) (Sloan 1959). Her
legibility ratings ranged from 0.90 (O) to 1.10 (Z).
Regrettably, many currently available charts, though using
sans serif letters, do not consider legibility ratings.
Legibility is also affected by ‘contour interaction’, a term
coined by Flom to describe the effect of neighbouring
contours on the discriminability of detail (Flom et al.
1963). Flom distinguishes between ‘contour interaction’,

AJ Jackson & IL Bailey
which affects resolution of detail, and ‘crowding’, which
affects assignment of attention. Both affect performance in
recognising optotypes on a visual acuity chart. Bailey
studied the effects of spacing between letters and between
rows for different optotypes. Using British Standard
letters, he found that a twofold increase in letter spacing
changed the visual acuity score by 0.03 log units (Bailey
1988). Spacing was noted to have more of an effect when
assessing acuity with Landolt rings than when using
Sloan-styled letters (Raasch & Bailey 1984). Bailey
recommends letter and row spacings should equal letter
width.
Row progression
Superimposed on the debate surrounding letter style has
been an equally vociferous debate about chart design and
row progression ratios. Both geometrical and arithmetical
progressions, using a variety of progression rates, have
been utilised over time. Snellen chose for his original
chart seven rows of letters which subtended 5min of arc at
distances of 6, 9, 12, 15, 21, 30 and 60m. The 1911 edition
of his chart changed the size progression to 6, 8, 12, 18,
24, 36 and 60-m letters. The six smaller rows approximate
a geometric or logarithmic progression with a multiplier of
1.4 (1.4 = √2 = 10 0.15 = 0.15 log unit). Using this factor, letter
size doubles every second row (6, 8.5, 12, 17, 24, 34).
Figure 5 illustrates the consequences of using three
different size progression sequences. One has a geometric
(logarithmic) progression with a constant multiplier ratio
of 10 0.1 (6/6, 6/7.5, 6/9.5, 6/12 ...), the second uses a simple
arithmetic progression in height (6/6, 6/12, 6/18, 6/24 ...)
and the third uses equal steps of the reciprocal of size with
0.1 increments on the decimal scale. The sequence on the
decimal scale is 1.0, 0.9, 0.8, 0.7 ... with the corresponding
Snellen sizes being (6/6, 6/6.7, 6/7.5, 6/8.6, 6/10 ...): the
figure is scaled so that the increments are equal on the
geometric scale. Three steps correspond to a twofold
change in size over the entire range. With the arithmetic
scaling sequence, three steps cover a four-times change in
size at the small end, and a 1.43 times change in size at
the large end. For the reciprocal progression, three steps
at the small end represent a 1.43 change in size, whereas
at the larger end of the scale, three steps cover a four
times change in size. Also shown on this figure is the
‘conventional’ size series commonly used in the UK (6/4,
6/5, 6/6, 6/7.5, 6/9, 6/12, 6/18, 6/24, 6/36 and 6/60). This
series uses optotypes tightly spaced at the smaller sizes
and more sparsely spaced at the larger sizes.
Westheimer (1979) took advantage of the eye’s natural
decline of visual acuity away from the fovea to show that
the ‘probability of seeing’ function, a graph showing
58
Increments on
visual acuity scales
Geometric
Reciprocal
Arithmetic
‘Conventional’
6/6
6/60
Figure 5 This figure shows the distribution of increments
on the visual acuity charts and their dependence on the
mathematical system by which the size increments are
determined.
The diamonds illustrate increments that follow a geometric
or logarithmic size progression. There is a constant ratio
between the sizes in adjacent rows. It is now widely
accepted that this system is the most appropriate for visual
acuity scaling. It is the basis for the log MAR and visual
acuity rating scales. Here, there are 10 steps between 6/6
and 6/60.
The open circles show the increments on a scale that
follows the reciprocal of visual angle. This is the basis of
the decimal visual acuity scale. The increments are
relatively compacted at the small size and sparsely spaced
at the larger size. Here there are nine steps between 6/6 and
6/60. The sequence is 1.0, 0.9, 0.8, 0.7, etc (6/6, 6/6.66,
6/7.5, 6/8.57, 6/10, 6/12, 6/15, 6/30, 6/60).
The open squares show the scale for an arithmetic
progression. This scale has equal steps in terms of
minimum angle of resolution (1, 2, 3, 4, 5, etc in minutes of
arc) or in terms of the Snellen fraction denominator (6/6,
6/12, 6/18, 6/24, 6/30, 6/36, 6/42, 6/48, 6/54, 6/60). The
increments are quite sparse at the smaller sizes and
compacted at the larger size. Here, there are nine steps
between 6/6 and 6/60.
The filled triangles illustrate the ‘conventional’ scale most
widely used in the UK. The increments are most densely
packed at the smaller sizes and more sparsely spread at
the larger size. There are nine steps between 6/6 and 6/60,
the sequence being (6/6, 6/7.5, 6/9, 6/12, 6/18, 6/24, 6/36,
6/60).

percentage correct recognition as a function of size, was
essentially the same for all levels of acuity, provided that
visual acuity was plotted on a logarithmic scale. There is a
broad acceptance that logarithmic (constant-ratio) scaling
should be used for visual acuity measurement, and the
most popular scaling ratio is now 0.1 log units (100.1 ,
1.2589 or approximately 3
√2). This progression, which was
proposed by Green (1905) and endorsed by the American
Committee on Optics and Visual Physiology (Ogle 1953;
Spaeth et al. 1955), has had more recent advocates in
Keeler (1956), Sloan (1951) and Bailey & Lovie (1976).
On this scale, 10 steps represent a 10-fold change, three a
twofold change, and one step represents a change of about
5/4. This sequence, based on a 6m reference, is 6, 7.5, 9.5,
12, 15, 19, 24, 30, 38, 48 and 60. For 20ft, the sequence is
20, 25, 32, 40, 50, 63, 80, 100, 125, 160, 200.
Test distance
The original Snellen chart was designed for use at a testing
distance of 20 Parisian feet (6.5m). (One Parisian inch is
equivalent to 1.0658 English inches.) By 1875, new
editions of the Snellen chart were calibrated in metric
units and were produced for use at 6m. Charts for use at
5m testing distances were, however, soon to become
available. Both Landolt and Monoyer advocated the use of
the 5m test distance (Landolt 1899, Monoyer 1875). The
XI International Ophthalmologic Congress in 1901 ratified
a recommended working distance of 5m (cited in Bennett
1965). Historically, the UK has stayed with a 6m standard
test distance, whereas in the USA the 20ft standard has
been adopted. Given that clinical rooms are rarely 6m
long, test distances are often achieved using indirect
charts viewed in a mirror. The US Committee on Vision of
the National Academy of Science/National Research
Council recommended that 4m be the standard test
distance (NAS-NRC 1980). Current British standards state
that the testing distance should not be less than 4m (BSI
4274-1 2003).
In low-vision work, the concept of a variable working
distance is widely used and charts may be presented at
various distances, sometimes as close as 50cm from the
eye. At close distances, attention may need to be paid to
the accommodation demand induced by reducing the
working distance. The dioptric depth of field of the eye
provides a tolerance to small errors of focus. It is generally
assumed that the refractive correction should be no more
than +0.25D from the optical correction that would be
optimal for the viewing distance that is being used. For
low-vision patients with poor visual acuity, wider dioptric
tolerances are often allowed. When testing younger
59
Visual Acuity
patients, with ample accommodative reserves, attention
may need to be paid to the potential for defocus induced
by proximal accommodation if the chart is at a close
distance.
Acuity designation
Whereas visual acuity is an expression of the angular size
of the smallest detail just resolvable by the eye, there are
several alternative ways of specifying the value clinically.
The Snellen fraction expresses the angular size of the
optotype as a fraction, specifying the test distance d as the
numerator and the distance at which the just-resolvable
letters should be positioned in order to subtend an angle
of 5min of arc as the denominator D (visual acuity = d / D)
(Figure 6). A benefit of expressing visual acuity in this way
is that the clinician records the testing distance, a factor
that can be important in paediatric eye care and lowvision
work. At a viewing distance of 6m, a patient with a
best corrected visual acuity of 6/24 will just be able to read
the 24m letter, which subtends 5min of arc at a distance
of 24m. In the USA, test distances are expressed in feet,
whereas in most European and Commonwealth countries,
a metric notation is used (6/6 = 20/20, 6/60 = 20/200, etc).
On the European continent, following the notation used
by Monoyer (1875), the Snellen fraction is usually
reduced to a decimal notation. An acuity of 6/6 thus
becomes 1.0, whereas a visual acuity of 6/60 becomes 0.1
in the decimal notation. With the decimal notation,
information on test distance is lost. Further, and more
importantly, visual acuity scores in decimal notation can
become confused with acuity scores expressed as log MAR.
Those familiar with the task of checking European and
Eye with 6/6
Eye with 6/18
Angle = 5 min of arc = 1/12 degree
E
6m 12m 18m
E
E E
E
6m 12m 18m
Figure 6 The Snellen fraction (e.g. 6/6, 6/12, 6/18, etc.), of
which the first number (numerator) indicates the test
distance in metres and the second number (denominator)
indicates the height of the letter. The height of the letter is
indicated by the distance at which a letter of that height will
subtend a specific angle, namely, 5min of arc = 1/12 degree.

AJ Jackson & IL Bailey
North American reference articles for comparable studies
will be familiar with the problems which can arise.
Alternative notations have been proposed by various
authorities over the years. Blaskoviscs (1924) proposed
the ‘oxyoptre’, which was the reciprocal of the visual angle
(in degrees) of the smallest recognisable target. Javal
(1900) proposed that ‘letter height’ should be used,
whereas Swaine (1925), who coined the term ‘visual
badness’, suggested that the Snellen fraction should be
inverted.
Minimum angle of resolution
MAR is used to express, usually in minutes of arc, the
angular size of the critical detail within a just-resolvable
optotype. It is usually assumed that the critical detail is
one-fifth of the letter height. Thus, an individual with a
best corrected visual acuity of 6/6 can resolve critical
detail, represented by a letter limb width of 1min of arc. If
the best corrected acuity were 6/60, the minimal limb
width resolvable would be 10min of arc. The MAR is, in
fact, the reciprocal of the decimal acuity value. The log
MAR designation was introduced in 1976 with the
Bailey–Lovie chart (Bailey & Lovie 1976). It expresses the
visual acuity as the common logarithm of the MAR. On
this scale, the 6/6 acuity (MAR = 1) becomes 0 on a log
MAR chart (log 10 (1.0) = 0). A 6/60 acuity (MAR 10)
becomes log 10 (10.0) = 1.0 (Table 2). Using charts with
five letters per row and a 0.1 log unit progression of size, a
Table 2. Comparison of different visual acuity designations.
Snellen
fraction
(metres)
Snellen
fraction
(feet)
Decimal
notation
Minimum angle
of resolution
(MAR)
Log MAR
Visual acuity
rating
(VAR)
60
value of 0.02 log MAR units can be assigned to each letter
read correctly. Other forms of acuity expression have also
been advocated. Bailey (1988) suggested the use of a
visual acuity rating (VAR), derived from log MAR acuity
values (VAR = 100 – 50 log MAR). Using this system 6/6
equates to VAR = 100, 6/60 becomes VAR = 50 and 6/600
becomes VAR = 0. There is one VAR point per letter on a
Bailey–Lovie or Early Treatment of Diabetic Retinopathy
Study (ETDRS) chart. Another designation system, visual
efficiency (VE), rarely used in the UK, was proposed by
Snell and Sterling in 1926 in an attempt to quantify visual
loss for medicolegal purposes (Snell & Sterling 1926).
They proposed the efficiency formula VE = 0.2
(MAR–1) /9 . An individual with ‘normal’ (20/20) vision
would be allocated a VE rating of 100%, while an individual
with a best corrected visual acuity of 20/200 would have a
VE rating of 20%. This method, which has been reviewed
by Hofstetter (1950), was adopted by the American
Medical Association in 1955 (Spaeth et al. 1955).
Somewhat similar ratings for field loss and motility
restrictions were subsequently developed and a
permanent impairment index was introduced in 1984
(American Medical Association 1984).
In many circles, within the UK at least, it has become
commonplace to record visual acuities of less than 6/60 as
CF, HM, PL or NPL. CF stands for count fingers and
indicates that the patient can recognise the number of
fingers held up by the clinician. This is an unnecessarily
Visual
efficiency
%
Conventional
Snellen Chart
Keeler
A chart
Snellen
fraction
(metres (4m))
Snellen
fraction
(feet)
In the conventional Snellen and Keeler A series columns, brackets around the acuity values indicate that the size is
only a moderately close equivalent to the values in the other seven columns. For the Keeler A series, exact Snellen
fraction equivalents are shown in parentheses.

crude way of estimating visual acuity. Finger widths will
vary from one clinician to another, separations between
fingers will differ from one presentation to the next and
the luminance and complexity of the background will
affect the visibility of the fingers, so the visual task is not
well-controlled. Working distances are often not recorded
by those who choose to express visual status in terms of
counting fingers. For patients whose visual acuity is less
than 6/60, appropriate measures of visual acuity may be
made by holding a visual acuity chart at a close distance.
If a 60m letter cannot be read at a distance of, say, 50cm
or even closer, then attempts at obtaining a meaningful
score of visual resolution may be abandoned.
Hand movements (HM) is not a measurement of visual
acuity, but rather a more basic and coarse classification of
visual function. It should be reserved for those cases
where the patient is unable to see any chart optotypes at
any distance. With this level of vision, the observer can
only detect shadows or large objects moving across the
visual field. Perception of light (PL) indicates an ability of
the eye to detect a light source which may be a ceiling
light, or an ophthalmoscope bulb held close to the eye.
Such a classification of vision should be reserved for eyes
incapable of detecting hand movements. It can be useful
to ask if the direction of the light source can be identified,
as an ability to judge the direction of major light sources
can be functionally valuable when it comes to orientation
awareness. The NPL (no perception of light) classification
of vision refers to total blindness.
Clinicians are often required to provide visual acuity
measurements in order to determine an individual’s
eligibility for benefits, for privileges or for occupational
assessment purposes. It can be useful to have information
on hand on the World Health Organization classification of
central visual status (World Health Organization 1979)
and local definitions that are relevant to occupational
standards (Association of Optometrists 2002), driving
(Taylor 1995) and visual impairment registration (Bunce
et al. 1998, Evans 1995).
Modern Test Chart Characteristics
Logarithmic progressions
The first of the modern logarithmically based test charts
was introduced to UK-based practitioners in 1956 by
Charles Keeler. The Keeler charts were referred to as A
series charts and were marketed as particularly useful for
those involved in low-vision practice (Keeler 1956). The
chart layout was generally similar to that of a conventional
Snellen chart. At the larger sizes, the sans serif letters had
61
Visual Acuity
stroke widths that were only one-seventh of the letter
height. An important feature was that size progression was
logarithmic. Successive sets of letters, moving up the
chart, were larger, by a factor of 1.25, than their
predecessors. On the original chart, there were eight A1
(6/6) letters and only one A20 (6/416) letter. In the USA,
Sloan developed a visual acuity chart using her family of
10 5 × 5 sans serif letters and a logarithmic size
progression with a multiplier ratio = 1.2589 = 10√10 =
10 0.1 (Sloan 1959). An important advance in the
measurement of visual acuity came with the development
and introduction of the Bailey–Lovie design principles
which were applied to new distance and near vision charts
in the 1970s (Bailey & Lovie 1976). The design principles
cause the visual acuity task to be standardised at each size
level. Other features that combined with the logarithmic
size progression to standardise the task are: (1) the same
number of optotypes in each row; (2) spacings between
optotypes and between rows are proportional to the size of
the optotypes; and (3) the mix of optotypes has
approximately the same level of difficulty. This means size
is the only significant chart variable from one size level to
the next. Their design uses a geometrical progression of
size and they chose a factor of 1.2589 (= 10 0.1 ) as their
multiplier ratio. Bailey and Lovie introduced the term ‘log
MAR’ as a measure of visual acuity used in conjunction
with a visual acuity chart. On their log MAR scale, there
are 10 steps between 6/60 and 6/6, with the size
progression being 60, 48, 38, 30, 24, 19, 15, 12, 9.5, 7.5,
6. On the charts, the log MAR values are in increments of
0.1 log units: 6/60 = 1.0 log MAR, 6/48 = 0.9 log MAR, 6/38
= 0.8 log MAR, and so forth (Table 2). Every third line on
the chart sees a doubling, for larger letters, or a halving,
for smaller letters, of letter size, so doubling or halving
causes a change in the log MAR value by 0.3. Each line
differs in size from the rows immediately above or below
by a factor of 1.2589 (0.1 log units). On currently available
charts, there is a 20-fold range of size with 14 rows of
letters which, assuming a 6m viewing distance, provide a
size range from log MAR 0.8 (6/38 equivalent) to log MAR
–0.5 (6/1.9 equivalent). On the Bailey–Lovie charts, there
are five letters per row, with the letters selected from the
group of 10 recommended in the 1968 British Standards
(BSI 4274-1 1968). The spacing between adjacent letters
is equal to one letter width, and the separation between
rows is equal to the width of the letters in the larger row.
The combination of letters on each line is selected to
ensure about equal average difficulty from row to row.
Visual acuity measurements recorded using Bailey–Lovie
charts can be determined using either row-by-row scoring
or letter-by-letter scoring. Common clinical practice is to
record the acuity as the smallest line on which the patient
is able to identify either 50% of 80% of letters correctly.

AJ Jackson & IL Bailey
This methodology is, however, too coarse to identify small
or moderate changes in acuity. A small change in the
number of letters read, even by one letter, can easily cause
the acuity score to change by one row. A change in visual
acuity score cannot be taken to be real or significant
unless the change is by two rows or more (Bailey et al.
1991). The row-by-row scores can be refined by applying
the plus or minus notations, commonly used when
expressing Snellen fraction measurements of acuity (6/6 ±
2). Giving credit for every letter read improves sensitivity
substantially (Figure 7). The Bailey–Lovie design
principles allow each letter to be given the same value in
log MAR or VAR units. Every extra letter read changes the
log MAR score by –0.02 and the VAR score by 1. This
facilitates scoring visual acuity letter-by-letter. With the
five-letters-per-row chart, each letter can be assigned a
value of 0.02 log MAR units (VAR = 1), with the result that
each additional letter read on a subsequent line changes
the log MAR score by –0.02 and the VAR by +1. The
patient who reads all letters on every row down to the 0.20
log MAR line (VAR = 90 = 20/32) and one letter on the 0.1
log MAR line would thus be allocated a log MAR score of
0.18 (VAR = 91). Another patient who reads all letters on
every row down to the 0.20 log MAR row, four out of five
on the 0.10 row and three out of five on the 0.00 row
scores, using individual letter scoring, a log MAR score of
0.06 (0.20 – 0.08 – 0.06 = 0.06). In VAR units the
equivalent score is 97 (90 + 4 + 3 = 97).
When giving credit for every letter read using a
Frequency
By row
By letter
Test / retest discrepancy (letters)
one row = five letters = 0.1 log unit
Figure 7 Distribution of test–retest discrepancies expected
when scoring visual acuity, recorded for normally sighted
subjects, on a Bailey–Lovie log MAR chart using
letter–by–letter and row–by–row scoring criteria. The 95%
confidence limit for change using letter–by–letter scoring is
±5 letters. For row by row scoring it is ± 2 rows.
62
Bailey–Lovie chart, a difference of five letters can usually
be taken as being significant. That is, there is a 95%
confidence that two visual acuity scores will be no more
than four letters different from each other when there has
been no change in vision. Thus a change of five letters can
be taken as being a real or significant change. Confidence
limits may be broader for some patient groups in whom
acuity measurements are less consistent (Bailey et al.
1991).
The term ‘log MAR chart’ is often used to describe charts
that follow the Bailey–Lovie design principles (Figure 8).
The most widely used log MAR chart is the ETDRS chart
designed for the Early Treatment of Diabetic Retinopathy
Study by Ferris et al. (1982). These charts, of which there
are three, use a combination of the 10 Sloan 5 × 5 letters
rather than the 5 × 4 British standard letters and they
were designed for use at 4m following the
recommendations of the Committee on Vision of the
National Academy of Sciences National Research Council
in the USA (NAS-NRC 1980). The use of standardised
illumination is encouraged and both direct and retro
illumination cabinets have been designed for use with
ETDRS charts (Ferris & Sperduto 1982).
Strong and Woo produced the Waterloo chart, which uses
a logarithmic progression, with letters in columns
becoming progressively smaller from left to right. They
also added contour bars at the end of each row and
Figure 8 A selection of currently available log MAR charts:
a high–contrast Bailey–Lovie chart, a low–contrast Bailey-
Lovie letter chart, an ETDRS chart, Glasgow Acuity cards
and Kay Crowded Symbol Cards.

column, thus ensuring that contour interaction affected all
letters equally (Strong & Woo 1985). Log MAR principles
have also been applied to the development of distance
visual acuity charts utilising Chinese characters (Woo &
Lo 1980). Similar principles have been applied to both
optotype and picture symbol acuity charts developed for
the assessment of vision in young children (Hyvarinen et
al. 1980, Jayatunga et al. 1995). McGraw and Winn
developed the Glasgow acuity card log MAR-based system,
which incorporates contour interaction bars, for assessing
vision in preschool children (McGraw & Winn 1993).
Recent studies indicate that visual acuity results recorded
using Kay picture, crowded symbol and crowded letter log
MAR-based tests, on children with amblyopia, are
comparable (Jones et al. 2003).
Other Test Chart Design Features
Historically, acuity charts were produced as printed
panels designed for use with direct illumination. Most
were printed on card and, as a result, were susceptible to
discoloration and ageing. Charts printed on plastic panels
were more durable and could, in addition, be wiped down
with a damp cloth, thus rendering them grime- and dirtfree.
Given the lack of availability of 6m testing facilities,
many visual acuity charts are printed in reverse format so
they can be viewed via a mirror in rooms 3–4m in length.
In an attempt to introduce standardisation of testing
conditions, internally illuminated acuity testing
equipment was produced, the most commonly used in
British hospitals being the rotating Snellen drum. Even
with these units, charts are subject to ageing and care
needs to be taken in the selection of background bulb
intensity. Log MAR charts are now also available as
internally illuminated units with interchangeable panels
to allow variation in the letter sequencing.
An alternative to the panel chart is the projector chart.
Typically their display area is relatively small in angular
size and this limits the length of the rows and the number
of print sizes that can be presented at the same time. The
angular size of the projected letters will be independent of
viewing distance provided the projection screen is
equidistant from both the projector and the patient. In
short rooms, mirrors may be used to increase the optical
path length from projector to screen and from screen to
patient. The magnification of the projector may be varied,
allowing precise calibration of the angular size of the
letters for a given eye-to-patient distance.
63
Test Chart Luminance
Visual Acuity
The sensitivity of the visual system is such that it is
capable of responding to illuminated targets over a very
wide range of intensities. The normal healthy eye can
operate over a very wide range (7–8 log units) of
luminance, encompassing both scotopic and photophobic
vision. Visual acuity does vary with changes in chart
luminance. The British Standard recommendations are
that internally illuminated charts should have a minimum
background luminance of 120 candela per square metre
(cd/m 2 ), with newly installed charts having a minimum
background luminance of 150 cd/m 2 in order to allow for
ageing of the system (BSI 4274-1 1968). Externally
illuminated charts should have a minimum illuminance of
480lux, although new installations should measure
600lux. Recommendations in the most recent British
Standards remain unchanged, although guidance on
background illumination has been withdrawn (BSI 4274-1
2003). The recommended lighting levels, for the
externally illuminated (807–1345lux) and internally
illuminated (343cd/m 2 ) charts advocated by Ferris, are
higher than those recommended in British Standards
(Ferris & Sperduto 1982). Sheedy and colleagues have
reported that, when chart luminance is within the
moderate photopic range (40–600cd/m 2 ), doubling the
chart luminance only alters the visual acuity score by a
little less than 0.02 log units (Sheedy et al. 1984). They
recommended that chart luminances be in the range
80–320cd/m 2 . Within any given clinical environment, it
has been recommended that chart luminance should be
kept constant with a tolerance of ±15% (Bailey 1998) and
that variance across a chart should not exceed 20% (BSI
4274-1 2003).
Testing Protocols
Many of the important principles and practices associated
with acuity testing have been outlined by Johnston
(1991). Irrespective of the type of chart used, or the test
distance selected, it is important to record both
monocular and binocular acuities. Good clinical practice
is to commence the assessment of visual functions by
recording habitual visual acuities with the patient
wearing his or her own distance spectacles or contact
lenses. Adapting the practice of recording data for the
right eye (Rt/OD) first, whilst ensuring total occlusion of
the left eye, minimises the risk of erroneously recording
laterally transposed data on the record card. Thereafter
left-eye data (Lt/OS) should be recorded using, if possible,
a separate acuity chart. This can be readily achieved using
a Snellen rotating drum system, hand-held charts or
alternatively by utilising different charts from the

AJ Jackson & IL Bailey
Bailey–Lovie or ETDRS sets. Generally, binocular acuities
(Binoc/OU) will be equal to or marginally better than best
monocular acuity. Sometimes, such as in the case of
congenital nystagmus, the difference may become as great
as 0.3 log MAR units. It is often useful to record unaided
acuities (Rt/Lt and Binoc), or ‘visions’, as they are often
called in the UK, in order to ascertain the extent to which
the refractive correction improves visual status. In cases
where an occupational report is requested, it is not
uncommon for employers to request information on both
habitual and unaided visual acuities as well as best
corrected acuities. Best corrected visual acuities,
recorded monocularly and thereafter binocularly, should
be recorded after full refraction. Pinhole acuities (PH) are
measures of visual acuity through a 1–1.5mm pinhole.
Pinhole acuities should be determined when the best
corrected visual acuity is less than expected or where
there is reason to suspect that visual impairment is
predominantly the result of medial irregularity as, for
example, in keratoconus or cortical cataract.
To ensure that a given patient’s threshold lies within the
range of the chart, all letters should be read at the largest
size and no letters should be legible at the smallest size. If
a patient correctly reads half or more of the letters in one
row, he or she should be encouraged to attempt to read
any letters in the following row. In clinical research
protocols, it is common practice to have all patients read
from the top of the chart, even for patients with excellent
visual acuity. This practice can be laborious when using
log MAR charts where, for normally sighted subjects, the
threshold size will commonly be at the 12th or 13th row
on the chart. In the interest of time, patients may be
encouraged to start reading three rows above the expected
threshold. Using log MAR charts, care has to be taken to
encourage patients with macular defects, and those with
neurological impairment, as the visual task can be difficult
and exhausting. Isolating letters, or sets of letters can be
useful and flip charts such as the Sonksen–Silver charts
may be useful in such circumstances (Sonksen & Silver
1988). For more expeditious testing, Rosser et al. designed
charts with an abbreviated size range, different letter
designs and fewer letters per row (Rosser et al. 2001). An
alternative approach advocated by Camparini and
colleagues is to estimate the threshold region by having
the patient read only the first letter in each row of ETDRS
charts until mistakes are made (Camparini et al. 2001). In
both cases, quicker testing is achieved with modest
reductions in reliability and validity.
There is a popular belief that 20/20 vision (6/6 or log MAR
= 0.0) represents ‘normal’ vision. Elliott and co-workers
have shown that the average best corrected visual acuity
in those under the age of 50, who show no signs of ocular
64
pathology, is about 6/4.8. Even in those over the age of 75
years, provided the eyes are free of disease, the average
visual acuity is marginally better than 6/6 (Elliott et al.
1995).
For clinicians to monitor changes in ocular health and
visual status, visual acuity measurements should be made
with appropriate test charts, viewed under controlled
conditions, and credit should be given to every additional
letter read correctly. The significance of this statement is
borne out by the publication of two British Medical
Journal papers on the subject in the mid-1990s (McGraw
et al. 1995, Pandit 1994). McGraw et al. concluded that
doctors must be extremely cautious when assigning
clinical importance to changes in acuity of two lines or
less because of the inherent variability of the Snellen
chart, whereas Pandit stressed the practical importance of
standardising working distances and illumination
conditions when assessing visual acuity using Snellen
charts in general practice.
The Impact of Glare
Light scatter from lenticular or corneal media haze
produces veiling luminance over the retina. This reduces
the contrast of the retinal image which produces glare
disability. Substantial visual disability and glare
discomfort may be encountered in certain lighting
conditions, such as when facing oncoming headlights
when driving at night, or when driving towards the setting
sun. In the consulting room, lighting conditions are rarely
sufficient to induce significant glare disability or
discomfort. Sometimes, vision may, however, be reduced
when reading material printed on glossy paper,
illuminated by an inappropriately positioned luminaire. A
clinical simulation of distance vision under glare can be
achieved by using the brightness acuity tester. This device
allows acuity to be assessed, viewing through a 12mm
diameter peephole in a 60mm hemispherical lightdiffusing
bowl which can be set at three different levels of
glare intensity. Neumann et al. (1988) found the
brightness acuity tester to be an accurate predictor of
outdoor high-contrast acuity in cataractous patients.
Glare has an even more dramatic impact on visibility
when the test targets are of low contrast (Elliott &
Bullimore 1993).
Contrast and Acuity Charts
Although a review of contrast sensitivity is outside the
scope of this paper, it should be acknowledged that
fundamental to vision is the eye’s ability to differentiate

luminance differences within an image formed on the
retina. The difference in luminance (∆L) that can just be
detected is proportional to the luminance of the
background on which the increment is displayed. This is
expressed as Weber’s law (∆L/L = constant), where L is the
overall background or prevailing luminance (Tunnacliffe
1994). The threshold contrast ratio (or Fechner constant)
is about 0.02–0.03 over the full range of photophobic
luminance (1–1000 cd/m 2 ). Under scotopic conditions, the
Fechner constant rises exponentially to 0.7 and the eye
becomes less good at detecting differences in luminance
levels within a target. When quantifying the contrast
inherent in a conventional black-on-white chart, it is
common practice to express contrast (C) as C = ∆L/L or
C = (L max – L min )/L max . The British Standard recommendations
are that conventional acuity charts should have a
contrast factor of 0.9 (90%), which is close to the
theoretical maximum with printed charts (BSI 4274-1
1968). Sloan suggested that the lowest acceptable contrast
ratio for the assessment of conventional high-contrast
acuity was 0.84 (84%) (Sloan 1951).
In recent years, increased understanding about the impact
of ocular and visual pathway pathology on contrast
sensitivity function has resulted in the development of
low-contrast acuity charts. Regan letter charts, which are
visual acuity charts with eight letters on each of five lines,
are available at 96%, 7% and 4% contrast levels. They are
designed for use at 3m and are scored on a letter-by-letter
basis (Regan 1988). Also available is the reduced-contrast
Bailey–Lovie visual acuity chart, which is identical to the
high-contrast chart in design, except that the letters are
printed at an 18% contrast level (Bailey 1982). The Regan
and Bailey–Lovie low-contrast visual acuity charts are
used to determine the smallest letters that can be read at
two or more different contrast levels. Less well-known are
the low luminance charts (Smith-Kettlewell Institute Low
Luminance or SKILL charts) designed by Haegerstrom-
Portnoy et al. (1997). These charts utilise black letters on
a dark-grey background and they have been shown to be
sensitive at detecting retinal and optic nerve disease.
A significantly different test is the Pelli–Robson lowcontrast
letter chart which measures contrast threshold,
not visual acuity. This chart consists of Sloan letters of
equal size (49mm) but of progressively decreasing
contrast. Letters are grouped in 16 sets of three, each set
decreasing in contrast by 0.15 log units. Those at the top
left-hand corner of the chart have 89% contrast whilst
those at the bottom right have 0.5% contrast (Pelli et al.
1988). This chart provides a contrast threshold
measurement at a single letter size.
65
Visual Acuity
Electronic Display Technology and Clinical
Measurement of Visual Acuity
Computer-controlled displays offer some important
advantages for clinical measurement of visual acuity and
related functions. For visual acuity charts, optotype
sequences may be varied, so memorisation becomes less
of a problem, especially when it is necessary to take
repeated measurements. Patients’ responses can be
recorded online and presentation protocols may be
modified to enhance efficiency of testing, reliability of
results and storage of data. Systematic variations may
easily be made in stimulus variables such as luminance,
contrast, spacing arrangements, optotype sizes, exposure
time, and so forth. Until recently it had been difficult to
obtain high luminances on electronic screens. Pixel
structure imposes some limitations in that, for reasonable
shape fidelity, letters need to be about 10 pixels or more
in height, and the need for more pixels may be more
pronounced for Landolt ring and tumbling E targets with
their more regular structures. The smallest letters on the
chart should be just beyond resolution for normally
sighted subjects.
Pixellation still imposes important limitations. If the
smallest letters on the chart are 10 pixels high, a row of
five 5 × 5 letters would need to be 110 pixels long, allowing
for one-letter spacing between letters, and similar spacing
at either end of the row. It would be 88 pixels for 5 × 4
optotypes. For a chart covering a 20-fold size range (6/60
to 6/3), the length of the largest row would thus need to be
2200 or 1760 pixels. With the separation between rows
being equal to the height of the letters, the height of the
chart would need to be at least 187 times the height of the
smallest letters for a 20-fold range of sizes. Thus, for a 6/60
to 6/3 size range, a display screen would need to have in
the order of 2000 pixels in both directions. The pixellation
constraints are not so restrictive in the vertical dimension
because vertical scrolling could be used to limit the
display to a size range of immediate relevance.
For letters to be 10 pixels high and to represent a 6/3
visual acuity angle the pixels need to subtend 15 seconds
of arc at the eye. This means the pixel size would need to
be 0.22mm for a 3-m viewing distance, or 0.44mm for 6m.
The screen width or height would thus need to be 45 or
90cm. Current display screen technology is very close to
being able to accommodate the high resolution and large
area required for a log MAR acuity chart with a size range
extending from 6/60 to 6/3. A particularly interesting
development is the BV-1000 automated subjective
refractive system, which has recently come to the market
in the UK. The system utilises single Landolt Cs oriented

AJ Jackson & IL Bailey
randomly in one of four directions. The optotype sizes
follow a logarithmic sequence and presenting visual acuity
levels range from 6/60 (log MAR =1.0) to 6/3.8 (log MAR =
–0.2). The single optotype presentation means the results
are not directly comparable to results from optotypes in
log MAR chart format (Dave 2003).
Acknowledgements
The authors wish to offer particular thanks to Miss E
Elliman, Mrs J Martin and Ms S O’Connor, whose
assistance in preparing the manuscript was invaluable.
Conclusion
Visual acuity, measured with letter charts, is likely to
remain the principal test of central visual function used by
clinicians. Advances in medical and optical technology
will inevitably lead to eye care practitioners being more
interested in accurately measuring small changes in
vision. Practitioners will need better measurements to
assess the outcome of sophisticated refractive surgery or
the benefits achievable through using new designs of
ophthalmic appliance. Decisions about treatments, access
to funds to pay for treatment, or eligibility for certain
benefits and privileges, will increasingly depend more on
clinical data, and tightly specified criteria, and less on a
clinician’s declaration of opinion.
The clinical community is slowly moving towards
improving visual acuity measurement. There is an
increased awareness that chart design affects visual acuity
scores. There is also a growing acceptance of the need to
standardize significant chart design features such as the
choice of optotype, the number of letters at each size, the
layout and spacing arrangements of the charts, and the
sequence of the size progression. Scoring visual acuity in
fine steps, giving additional credit for each additional
letter read, should become the rule rather than the
exception as clinicians become more interested in getting
more precise and reliable measures of vision.
As with the assessment of visual fields, progress in visual
acuity measurement, by real-world clinicians, is likely to
be gradual until such times as computer-based technology
becomes adequate and affordable. It will not be long.
When computer-based acuity measurement is made easy,
and incorporated into examination routines, clinicians
will embrace visual acuity tests with randomly generated
letter sequences, efficient automated procedures to
establish threshold, and automatic registering of precise
visual acuity scores with measures of reliability. Ready
66
access to controlling target characteristics, luminances
and layouts will enable clinicians to make more thoughtful
systematic evaluations of the visual capabilities of their
patients.
References
American Medical Association (1984) Evaluation of permanent visual
impairment. In: AMA Council on Scientific Affairs (eds) Guides to the
Evaluation of Permanent Impairment. AMA Physicians Desk
Reference for Ophthalmology, 2nd edn, pp. 141–51. Chicago
Association of Optometrists (2002) AOP Members’ Handbook 2002.
Section C Vision Standards. London: Association of Optometrists
Bailey IL (1982) Simplifying contrast sensitivity testing. Am J Optom
Physiol Opt 59 (suppl.),12
Bailey IL (1988) Measurement of visual acuity – towards
standardisation. In: Vision Science Symposium: A Tribute to Gordon
G Heath, pp. 217–30. Bloomington: Indiana University
Bailey IL, Bullimore MA, Raasch TW, Taylor HR (1991) Clinical
grading and the effects of scaling. Invest Ophthalmol Vis Sci 32,
422–32
Bailey IL (1998) Visual acuity. In: Benjamin WJ, Borish IM, Lampert
R (eds) Borish’s Clinical Refraction, ch. 10. Philadelphia: WB
Saunders
Bailey IL, Lovie JE (1976) New design principles for visual acuity
letter charts. Am J Optom Physiol Opt 53 (11), 740–5
Bennett AG (1965) Ophthalmic test types. Br J Physiol Opt 22,
238–71
Blaskoviscs L (1924) The new unit of visual acuity and its practical
use. Arch Ophthalmol 53, 476–85
Borish IM (1970) Clinical Refraction, Visual acuity, 3rd edn, pp.
345–422. Chicago, IL: Professional Press
BSI 4274–1 (1968) Visual Acuity Test Types Pt 1: Specification for
Test Charts for Clinically Determining Distance Visual Acuity.
London: British Standard Institute
BSI 4274–1 (2003) Visual Acuity Types Pt 1: Test Charts for Clinical
Determination of Distance Visual Acuity: Specifications. London:
British Standards Institute
Bunce C, Evans J, Fraser S, Wormald R (1998) BD8 certification of
visually impaired people. Br J Ophthalmol 82 (1), 72–76
Camparini M, Cassinari P, Ferrigno L, Macaluso C (2001) ETDRS –
fast: implementing psychophysical adaptive methods to standardise
visual acuity measurement with ETDRS charts. Invest Ophthalmol
Vis Sci 42 (6), 1226–31
Coates WR (1935) Visual acuity test letters. Trans Inst Ophthalm
Opticians III, 1–38
Cowan A (1928) Test cards for the determination of visual acuity: a
review. Arch Ophthalmol NY 57, 283–92
Dave T (2003) Topcon BV 1000 automated subjective refraction
system. Optom Today April 4th 1–6
Dennett WS (1885) Test type. Trans Am Ophthalmol Soc 4, 133–9
Duke-Elder SW (1937) The subjective examination of the eye. In:
Duke-Elder SW (ed.) Textbook of Ophthalmology, vol. II, ch. 29.
London: Henry Kimpton
Elliott DB, Bullimore MA (1993) Assessing the reliability,
discriminative ability and validity of disability glare tests. Invest
Ophthalmol Vis Sci 34 (1), 108–19
Elliott DB, Yang KCH, Whitaker D (1995) Visual acuity changes
throughout adulthood in normal healthy eyes: seeing beyond 6/6.
Optom Vis Sci 72 (3), 186–91

Evans J (1995) Causes of Blindness and Partial Sight in England and
Wales 1990–1991. Studies on Medical Population Subjects No. 57.
London: HMSO
Ferris FL, Sperduto RD (1982) Standardised illumination for visual
acuity testing in clinical research. Am J Ophthalmol 94, 97–8
Ferris FL, Kassoff A, Bresnick GH, Bailey IL (1982) New visual acuity
charts for clinical research. Am J Ophthalmol 94, 91–6
Flom MC, Weymouth FW, Kahneman K (1963) Visual resolution and
contour interaction. J Opt Soc Am 53 (9), 1026–32
Green J (1868) On a new series of test letters for determining
acuteness of vision. Trans Am Ophthalmol Soc 1 (3), 68–71
Green J (1872) Some Improvements in Test Letters. London: IVth
International Ophthalmology Congress
Green J (1905) Notes on the clinical determination of the acuteness
of vision, including the construction and gradation of opto-types and
on systems of notation. Trans Am Ophthalmol Soc 10 (3), 644–654
Haegerstrom-Portnoy G, Brabyn J, Schneck M, Jampolsky A (1997)
The SKILL card: an acuity test of reduced luminance and contrast.
Invest Ophthalmol Vis Sci 38 (1), 207–18
Hartridge H, Owen HB (1922) Test types. Br J Ophthalmol 6, 543–549
Hofstetter HW (1950) The AMA method of appraisal of visual
efficiency. Am J Optom Arch Am Acad Optom 27 (2), 55–63
Hyvarinen L, Nasanen R, Laurinen P (1980) New visual acuity test for
pre-school children. Acta Ophthalmol 58, 507–11
Javal E (1900) Réforme de la notation de l’acuité visuelle. Ann Oculist
124, 155–9
Jayatunga R, Sonksen PM, Bhibe A, Wade A (1995) Measures of acuity
in primary school children and their ability to detect minor errors in
vision. Dev Med Child Neurol 37, 515
Jenkins FA, White HE (1976) Fundamentals of Optics, 4th edn, p.
329. Kogakusha: McGraw–Hill
Johnston AW (1991) Making sense of the M, N and log MAR systems
of specifying visual acuity. Probl Optom (Lippincott) 3 (3), 394–407
Jones D, Westall C, Averbeck K, Abdolell M (2003) Visual acuity
assessment: a comparison of two tests for measuring children’s vision.
Ophthalmol Physiol Opt 23 (6), 541–6
Keeler CH (1956) Visual aids for the partially sighted. Trans
Ophthalmol Soc UK 76, 605–14
Landolt E (1899) Nouveaux opto-types pour la détermination de
l’acuité visuelle. Arch Ophthalmol Paris 19, 465–471
McGraw PV, Winn B (1993) Glasgow acuity cards: a new test for the
measurement of letter acuity in children. Ophthalmol Physiol Opt 13
(4), 400–3
McGraw P, Winn B, Whitaker D (1995) Reliability of the Snellen chart.
BMJ 310, 1481–2
Monoyer M (1875) Echelle typographique décimale pour mésurer
l’acuité de la vue. CR Hebd Séance Acad Sci Paris 80, 1137–8
NAS–NRC (National Academy of Science: National Research Council
Vision Committee on Vision) (1980) Recommended standard
procedures for the clinical measurement and specification of visual
acuity (report 39). Adv Ophthalmol 41, 103–48
Neumann AC, McCarty GR, Locke J, Cobb B (1988) Glare disability
devices for cataract: a consumer’s guide. J Cataract Refract Surg 14,
212–16
Ogle KN (1953) On the problem of an international nomenclature for
designating visual acuity. Am J Ophthalmol 36, 909–21
Pandit JC (1994). Testing acuity of vision in general practice: reaching
recommended standards. BMJ 309, 1408
Pelli DG, Robson JG, Wilkins AJ (1988) The design of a new letter
chart for measuring contrast sensitivity. Clin Vision Sci 2 (3),
187–200
67
Visual Acuity
Raasch T, Bailey I (1984) Choice of optotype and spacing affect visual
acuity scores. Invest Ophthalmol Vis Sci 25 (suppl. 86), 145
Regan D (1988) Low contrast letter charts and sinewave grating tests
in ophthalmological and neurological disorders. Clin Vision Sci 2,
235–50
Rosser DA, Laidlaw DAH, Murdoch IE (2001) The development of a
‘reduced log MAR’ visual acuity chart for use in routine clinical
practice. Br J Ophthalmol 85, 432–6
Sheedy JE, Bailey IL, Raasch TW (1984) Visual acuity and chart
luminance. Am J Optom Physiol Opt 61 (9), 595–600
Sloan LL (1951). Measurement of visual acuity. AMA Arch
Ophthalmol 45, 704–25
Sloan LL (1959) New test charts for the measurement of visual acuity
at far and near distances. Am J Ophthalmol 48 (6), 807–13
Sloan LL, Rowland WM, Altman A (1952) Comparison of three types
of test target for the measurement of visual acuity. Q Rev Ophthalmol
8, 4–16
Snell AC, Sterling S (1926) An experimental investigation to
determine the percentage relation between macular acuity of vision
and macular perception. In: Menasha G (ed.) Contributions to
Ophthalmic Science, pp. 52–62. Banta
Snellen H (1862) Letterproeven tot Bepaling der Gezigtsscherpte.
Utrecht: PW Vander Weijer
Snellen H (1864) Test Types for the Determination of the Acuteness of
Vision. Official War Office edition (2nd edn). Utrecht: PW Vander
Weijer
Sonksen PM, Silver J (1988) The Sonksen Silver Acuity System. Test
System and Instruction Manual. Windsor: Keeler
Spaeth EB, Fralick FB, Hughes WF, Scheie HG (1955) American
Medical Association council on industrial health: special report,
estimation of loss of visual efficiency. Arch Ophthalmol 54, 462–8
Stiles WS, Crawford BH (1933) The luminous efficiency of rays
entering the eye pupil at different points. Proc R Soc (Lond) Biol 112,
428–50
Strong G, Woo GC (1985) A distance visual acuity chart incorporating
some new design features. Arch Ophthalmol 103, 44–6
Swaine W (1925) The relation of visual acuity and accommodation to
ametropia. Trans Opt Soc 9–27
Taylor JF (ed.) (1995) Medical Aspects of Fitness to Drive. London:
Medical Commission on Accident Preventions
Tunnacliffe A (1994) Visual acuity updated: 4. Br J Optom Dispens 2
(12), 455–8
Walker JPS (1942) Test type. Br J Ophthalmol 26, 556–559
Weber EH (1834) De Pulsu, Resorptione, Auditu et Tactu
Annotationes Anatomicae et Physiologicae. Leipzig: CF Koehler
Westheimer G (1964) Pupil size and visual resolution. Vis Res 4, 39–45
Westheimer G (1979) Scaling of visual acuity measurement. Arch
Ophthalmol 97, 327–30
Woo G, Lo P (1980) A Chinese word acuity chart with new design
principles. Singapore Med J 21, 689–92
Woodruff EW (1947) Visual acuity and the selection of test letters. In:
Some Recent Advances in Ophthalmic Optics, pp. 59–70. London:
Hatton Press
World Health Organization (1979) Guidelines for Programmes for the
Prevention of Blindness. Geneva: World Health Organization

AJ Jackson & IL Bailey
Multiple Choice Questions
This paper is reference C5333b. Four College credits are available. Please use the inserted answer sheet. Copies can be obtained
from Optometry in Practice Administration, PO Box 6, Skelmersdale, Lancashire WN8 9FW. There is only one correct answer for
each question.
1. The diffraction pattern of the image of a point source
of light
(a) Has a Gaussian profile.
(b) Consists of a series of concentric circles.
(c) Has a central disk surround by rings.
(d) Is independent of wavelength.
(e) Is independent of pupil size.
2. Monochromatic light from a point object, focused by
the optics of the eye, forms a patch of light on the
retina. The light distribution across the patch is
referred to as the
(a) Ramsden disk.
(b) Petzval surface.
(c) Interval of Sturm.
(d) Contrast sensitivity function
(e) Point spread function.
3. In a system free of aberrations or defocus, visual
resolution
(a) Is poorer with smaller pupils.
(b) Is better with smaller pupils.
(c) Is independent of the wavelength of light.
(d) Is better when the point spread function is larger.
(e) Is better if the areas of the foveal cones are larger.
4. Under photopic conditions, the differential contrast
threshold remains a constant ratio of approximately
(a) 80%
(b) 20%
(c) 15%
(d) 1%
(e) 0.1%
5. For a 3-bar target, spurious resolution
(a) Causes vertical lines to appear horizontal.
(b) Causes noticeable colored fringes.
(c) Causes a doubling of the spatial frequency.
(d) Causes the 3 lines to appear to be 2.
(e) Causes the 3 lines to appear to be 4.
6. Hyperacuities
(a) Occur only for sinusoidal grating targets.
(b) Occur only under binocular viewing and bi-foveal
fixation.
(c) Occur because of vertical orthophoria.
(d) Occur because of pooling information from the two
eyes.
(e) Occur because of pooling of spatial information
across receptors.
68
7. Regarding optotypes for visual acuity measurement
(a) British Standard letters are all 5x4 letters with serifs.
(b) Sloan letters are all 5x5 letters without serifs.
(c) Keeler chart letters all have stroke widths that are
20% of the letter height.
(d) Landolt’s Rings were developed before Snellen’s
Letters.
(e) Snellen’s original letter set were all 5x5 letters.
8. The term “Oxyoptre” as suggested by Blaskovics is a
measure of
(a) Retinal image blur in diopters.
(b) Optical defocus.
(c) The reciprocal of the visual angle in degrees.
(d) Spatial frequency.
(e) Longitudinal chromatic aberration in diopters.
9. Regarding the designation of visual acuity
(a) The Snellen fraction is a measure of letter height.
(b) The MAR is the angle that the letter (or optotype)
height subtends at the eye.
(c) If a chart follows the Bailey-Lovie format, each letter
has a value of 0.02 logMAR units.
(d) LogMAR notation can only be used if there is the
same number of letters on each row.
(e) In decimal notation, 2.0 is equivalent to 6/12.
10. Which of the following is NOT true about charts that
follow the Bailey Lovie principles
(a) The progression of size should follow a constant
ratio.
(b) There should be the same number of letters (or
optotypes) per row.
(c) The letters (or optotypes) chosen for each row
should be balanced for difficulty.
(d) Between-row spacing should be proportional to letter
height.
(e) Between-letter spacing should be proportionally
greater when letter sizes are smaller.
11. Comparing the merits of row-by-row and letter-byletter
scoring methods
(a) If using letter-by-letter scoring, you are more likely
to get exactly the same score for test and retest.
(b) If using row-by-row scoring, you are less likely to
have 1 row difference between test and retest.
(c) If using row-by-row scoring, you are less likely to
have 2 rows difference between test and retest.
(d) If using letter-by-letter scoring, you ignore errors
made at sizes larger than the smallest row on which
more than 50% of the letters were read correctly.
(e) If using letter-by-letter scoring, you are substantially
more able to identify changes in visual acuity.
Questions for this paper are continued on inside back cover